Boreholes are created by drilling into the earth using a rig. The rig drives a bottomhole assembly (BHA) on a drill string to create a hole. The BHA comprises a drill bit, which is provided with sufficient weight-on-bit (WOB) to break the rock. The BHA also may provide directional control of the drill bit and may use sensors to take downhole measurements of actual drilling conditions.
Drilling fluid or drilling mud is pumped downhole through a drill pipe while drilling. The drilling fluid cools the drill bit, circulates through the borehole, and returns drill cuttings, such as sand and shale, to the surface. The cuttings are passed through a shaker which strains the cuttings from the drilling fluid, and optionally, through a centrifuge which separates cuttings such as sand from the drilling fluid. The cleaned drilling fluid is then returned downhole through the drill pipe.